A polymer having a hydrophilic group such as a sulfonic acid group, a carboxyl group, and a phosphoric acid group, that is, a polymer electrolyte is utilized as a proton conducting membrane of a fuel cell, or a binder for complexing with platinum-supported carbon to form a catalyst layer. As such the polymer electrolyte include Nafion (registered trade mark) manufactured by Dupont, Aciplex (registered trade mark) manufactured by Asahi Chemical Industry Co., Ltd., and Flemion (registered trade mark) manufactured by Asahi Glass Company, and these are sold in a form in which a solid microparticle consisting of a fluorine-based polymer electrolyte is dispersed in a dispersing medium. However, since such the fluorine-based polymer electrolyte has been desired to be substituted with a polymer electrolyte due to request of discharge of a fluorine compound generated by degradation to the environment, and the lower cost.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2005-132996 discloses an aqueous dispersion comprising sulfonated polyorganosiloxane, and indicates that such the dispersion imparts a film excellent in water resistance and film forming property.
In addition, JP-A No. 2004-319353 discloses that a solid electrolyte membrane obtained by mixing a solid electrolyte consisting of a fine particle having an ionic group on a surface with other polymer electrolyte soluble in a solvent, and casting the solution affords a solid electrolyte membrane which suppresses methanol crossover of a direct methanol fuel cell.